This invention relates to a reclining device wherein internal and external gears having different numbers of teeth are journalled on a common eccentric shaft and a differential reduction gear adapted to rock as the eccentric shaft rotates is employed to make a vehicular seat comfortable for seating.
One prior art reclining device employing a differential reduction gear is shown in FIG. 1. In the prior art reclining device shown in FIG. 1, the internal gear 1 is formed integrally with the bracket 2 adapted to be secured to the seat cushion, the external gear 3 is formed integrally with the bracket 4 adapted to be secured to the seat back and the internal and external gears 1 and 3 are rockably journalled on the eccentric shaft 5 while the internal and external gears are always maintaining their partially engaging condition.
Thus, when the eccentric shaft 5 is rotated, the relative position of the brackets 2, 4 varies steplessly by an angular amount corresponding to the difference in the numbers of teeth on the internal and external gears 1 and 3 to thereby perform adjusting of the reclining angle of the vehicular seat.
In the prior art reclining device described above, the internal and external gears 1 and 3 journalled on the eccentric gear 5 are indirectly and partially pinched by the bearing bracket 6, flanged pin 7 and flanged bushing 8 or partially secured to the bearing bracket, flanged pin and flanged bushing. Thus, since stresses such as impact load and twist load tend to concentrate on the internal and external gears, the gears are easily damaged. Thus, means for reinforcing the gears have to be devised. And in the prior art reclining device, the displacement of the gears in the axial direction (the horizontal plane) of the eccentric gear or oscillation of the gears 1 can not be positively prevented and the internal and external gears may not properly mesh with each other due to error occured in mounting the gears whereby the gears can not operate smoothly.
And when a small size and high speed motor or the like is employed as the drive source of the reclining device, it is advantageous that large size internal and external gears having large numbers of teeth be provided. However, in the above-mentioned prior art reclining device wherein the internal and external gears are indirectly and partially pinched by the bearing bracket, flanged pin and flanged bushing, when the internal and external gears are large in size, even if the internal and external gears are out of alignment slightly, the tips of the teeth on the gears are substantially out of alignment whereby the teeth on the gears tend to easily disengage from each other resulting in reduction in the strength of the gears.
Furthermore, the output shaft of a small size motor is small in diameter and low in supporting strength in the axial direction. And there is the drawback that when the output shaft of the motor is subjected to thrust load from a driven member, the motor shaft gets damaged. In addition, the small size motor requires a reduction gear having a large reduction ratio and oscillation noise of high frequency are produced from the motor and reduction gear to give uncomfortable noise and oscillation to the occupant to thereby make it difficult to devise counter measures for prevention of oscillation and noise.